Over the last decade, baud rate of digital optical signal in data communication has seen dramatic increase from around 10 Gbaud, to 28 Gbaud, and to 56 Gbaud. Not surprisingly, even higher baud rates are forth coming. Optical data communication involves electro-optic signal conversion. However, one of the main challenges facing this ever increasing baud rate is how to effectively and efficiently handle signal connection between an optical sub-assembly (OSA), which houses various modules that handle electro-optic signal conversion, and a main printed circuit board (PCB) which handles digital signal processing in the electrical domain.
The OSA provides optical to electrical (O/E) and/or electrical to optical (E/O) signal conversion, which are generally referred to as O/E or E/O conversion within the industry. For example, an OSA that handles electrical to optical signal conversion is generally referred to as a transmitter OSA (TOSA) while an OSA that handles optical to electrical signal conversion is generally referred to as a receiver OSA (ROSA). Both TOSA and ROSA are key components or sub-assemblies inside an optical transceiver that is widely used in equipment of data center and telecommunication service provider's network system. Optical transceivers that adopt small form factor (SFF) are a corner-stone of modern data centers. SFF may include, for example, SFP, QSFP, and OSFP.
An OSA (TOSA and ROSA) typically includes components such as laser diode (LD), photo detector (PD), planar lightwave circuits (PLC) and/or free-space based optical mux/demux, and electronic integrated circuits (IC's), such as laser driver, trans-impedance amplifier (TIA), and so on. As is simplistically illustrated in FIG. 7(a), and according to current art, a TO-CAN or box-based OSA 11 generally includes one or more of the above components contained in its own package, which is connected to an outside main PCB 10 through two transitional connections 13 and 14. A first transitional connection 13 is made from the OSA 11 to a flexible printed circuit (FPC) 12. The FPC 12 is then connected to the main PCB 10 through a second transitional connection 14. In other words, a FPC is used as a transitional medium to interconnect the OSA 11 with the main PCB 10. The two connections 13 and 14 made at the OSA end and the main PCB end are commonly through soldering or epoxy. Being a separated part both to the OSA and to the main PCB, the FPC provides a flexible connection between OSA 11 and main PCB 10. The use of a separate FPC is important in order to increase product yield and reliability that are often associated with the mechanical manufacturing tolerance of each components.
On the other hand, with the baud rate of digital data transmission going above 20 Gbaud, the effect of signal degradation caused by the two transitional connections 13 and 14 (between the OSA and the FPC and between the FPC and the main PCB) becomes more and more critical and sometimes becomes simply intolerable by the system. For example, for baud rate of digital signal of data transmission that is 50 Gbaud and beyond, due to manufacturing limitation in precision tolerance of the OSA, FPC, and PCB modules, and their assembly through soldering, impact on signal degradation brought upon by the above interconnect approach are so significant that they can no longer be ignored.
To mitigate the above concerns of signal degradation, so far the direction of industry is heading toward soldering (or through the use of epoxy) the OSA directly onto the main PCB, bypassing the use of the traditional FPC as a transitional medium, as is simplistically illustrated in FIG. 7(b). Under this situation, electrical signals associated with the OSA 21 are connected through wire bonding 22 directly to the main PCB 20. However, this approach comes with the disadvantage that after the OSA-PCB assembly is finished, the OSA 21 can no longer be easily removed from the main PCB 20 for rework or repair, particularly after the encapsulation 23 is added onto the OSA portion 21 in order to protect bonding wire 22 and prevent humidity effect. Consequently, this approach of directly wire-bonding OSA module onto main PCB brings with it significant impact on the overall production yield.